The invention relates to an electrophotographic copying machine, and more particularly, to an electrophotographic copying machine of the type which is commonly referred to as moving original type in which a sheet original or a thick original placed on a book carrier is fed over an exposure window for copying purpose.
U.S. Pat. No. 3,659,937, assigned to Minolta Camera K.K., discloses an electrophotographic copying machine of moving original type in which an original in the form of a sheet is conveyed over an exposure window while it is closed by a cover while when copying from a thick original in the form of a book or the like, the cover is moved to an open position where it clears the exposure window to define a conveying path for a book carrier on which the thick original is placed. Such copying machine utilizes a dielectric record medium, and the sequential steps of charging, exposure, developing and fixing are directly applied to each single sheet of such record medium without causing any difficulty.
However, a problem is presented with an electrophotographic copying machine as disclosed in U.S. Pat. No. 4,171,157, assigned to Olympus Optical Co., of moving original type in which a photosensitive drum is used to provide a plurality of copies from a single exposure step, in that the incidence of extraneous light into the machine through the exposure window may be allowed immediately after the book carrier has moved past the window in the absence of any light shield which closes the window. The extraneous light erases the electrostatic latent image formed on the drum, preventing a subsequent copying operation which is utilized to provide a plurality of copies. An exemplary construction of copying machine of this type will be described more fully with reference to FIG. 1. In the electrophotographic copying machine illustrated, a cover 35, which also serves as an extension frame of a conveying path to be described later, normally closes the region above an exposure window 8, and defines an original conveying path for allowing a copying operation from a single original in the form of a sheet. Under this condition, when a sheet-shaped original 1 is placed on an inclined original guide 2 and inserted into the inlet of an original feeder comprising conveying rollers 4A, 4B, 5A, 5B and guide plates 6, 7, in a direction indicated by an arrow A, it is held between the pair of vertically aligned conveying rollers 4A, 4B to be fed toward the exposure window 8. After passing over the exposure window 8, the original 1 is held between and conveyed by another pair of vertically aligned conveying rollers 5A, 5B onto an original tray 9.
As the original 1 is fed by the original feeder, a pair of microswitches 3A, 3B located on the opposite sides of the conveying rollers 4A, 4B detect the location of the original 1 and provide an output which is utilized for timing the operation of various parts of the machine. As the original 1 moves over the exposure window 8, the original surface is illuminated by light from an illumination lamp 10, and the image of the original is projected onto a photosensitive drum 12 through Selfoc (trademark) optical member 11, which represents a focussing optical fibre having a continuously varying refractive index. The drum 12 rotates in a direction indicated by an arrow B and is initially charged in a uniform manner by a corona charger 14 before it is irradiated with the light image of the original. In this manner, an electrostatic latent image of the original 1 is formed on the drum surface. The latent image is converted into a toner image by a developing unit 15 of the dry type, and a toner image is carried to a transfer station 16 as the drum 12 continues to rotate.
A cassette 17 contains a stack of record sheets 18 which are formed of a plain paper, and which are fed one by one by an oscillating and rotating feed roller 19 and then fed by a pair of vertically aligned feed rollers 20 to the transfer station 16 in timed relationship with the rotation of the drum. At the transfer station 16, the record sheet is fed into overlapping relationship with the toner image on the drum by being passed between the drum and transfer roller 21 to which a bias voltage is applied, thus transferring the toner image onto the record sheet. Subsequently, the record sheet which is held tight against the drum 12 is separated therefrom by the action of a separating claw 22 and an airstream to be described later. Then the record sheet having the toner image transferred thereon is conveyed along a guide 23, and a pair of vertically aligned conveying rollers 24 convey it into a fixing unit 25 comprising a heater where the toner image is fixed to the sheet by melting. Subsequently, the sheet is delivered onto a copy tray 27 by another pair of vertically aligned delivery rollers 26.
Any residue of toner which remains on the drum surface without being transferred onto the record sheet is scraped off by a rotating cleaning brush 28 and withdrawn into an airstream created by a fan 29 to be collected by a filter 30. Both the cleaning brush 28 and the fan 29 are surrounded by a casing 31 which is effective to prevent the toner from being dispersed into the machine and to provide an effective pumping action upon the residual toner. The airstream withdrawn by the fan 29 is led into a duct 32 having an outlet port 32a located adjacent to the transfer station 16 so as to be utilized in the successive separation of the record sheet from drum 12 by cooperating with the separating claw 22.
The copying machine described above is adapted to provide a plurality of copies by a repeated use of the electrostatic latent image which is once formed on the drum 12 through the performance of repeated developing and transferring steps. When used in this mode of providing a plurality of copies from the single exposure, the cleaning brush 28 is moved away from the drum 12. To achieve this, brush 28 is mounted on a support 34 which is pivotally mounted on a pin 33. Also, a nuetralizing lamp 13 and charger 14 must be deactivated.
While the copying machine described above is principally used in copying from a single original in the form of a sheet, it may also be used in copying from a thick original in the form of a book or the like. In this instance, the cover 35 which carries the upper conveying rollers 4B, 5B and the guide plate 6 is turned through an angle of 180.degree. in a direction indicated by an arrow C about a pin 36 so that the conveying rollers 4B, 5B and guide plate 6 form an extension of the conveying path which is contiguous with plane of the exposure window 8 and located above the original tray 9 (see FIG. 2).
When copying from a thick original such as a book under this condition, use is made of a book carrier on which the thick original is placed. The book carrier with the thick original thereon is initially placed on the original guide 2, and is conveyed over the exposure window 8 by conveying rollers 4A, 5A which are drive rollers, during a copying operation. As the book carrier moves past the exposure window 8, the original surface is exposed through a transparent plate on which the book is placed, thus allowing a copying operation from the book.
As mentioned previously, when the described copying machine is used in the mode which permits a plurality of copies to be obtained from the single exposure, the electrostatic latent image once formed on the drum 12 is used repeatedly, thus allowing the toner developing and transfer steps to be repeated subsequently. For this reason, after the latent image is formed, the presence of the original is no longer required, permitting the original to be removed or replaced by another original during the time a desired number of copies are being produced.
However, in actuality, if the original is removed from its position over the exposure window 8 during the time the number of copies are being produced, extraneous light which has been interrupted by the original is allowed to impinge into the interior of the machine through the window 8, thus causing a degradation of the latent image formed on the drum 12. In particular, when the book carrier is utilized to permit a copying from a thick original, the cover 35 is not in a position to close the exposure window 8 as shown in FIG. 2, so that if the book carrier is moved into or out of the position covering window 8, extraneous light 37 as indicated by an arrow impinges on the optical member 11 through the exposure window 8, thus causing the latent image formed on the drum surface to be degraded.